1. Field of the Invention
The present invention relates generally to a wireless communication system, and more particularly, to a method and apparatus for blindly detecting a common search space and a User Equipment (UE) specific search space.
2. Background of the Invention
In a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system, the length of each radio frame is 10 ms, which is divided equally into 10 subframes. One downlink Transmission Time Interval (TTI) is defined on one subframe. FIG. 1 is a diagram illustrating a frame structure in a Frequency Division Duplex (FDD) system. Each downlink subframe includes two time slots. For a normal Cyclic Prefix (CP) length, each time slot includes 7 Orthogonal Frequency-Division Multiplexing (OFDM) symbols. For an extended CP length, each time slot includes 6 OFDM symbols.
FIG. 2 is a diagram illustrating a frame structure in an LTE system. The first n OFDM symbols, where n is equal to 1, 2 or 3, are used for transmitting downlink control information, including a Physical Downlink Control CHannel (PDCCH) and other control information. Remaining OFDM symbols are used for transmitting a Physical Downlink Shared CHannel (PDSCH). A granularity of resource allocation is a Physical Resource Block (PRB). One PRB includes 12 consecutive sub-carriers in frequency and corresponds to one time slot in time. Two PRBs in two time slots of the same sub-carrier in one subframe is referred to as a PRB pair. In each PRB pair, each Resource Element (RE) is a minimum unit of time and frequency resources, i.e., the RE includes one sub-carrier in frequency and includes one OFDM symbol in time. REs may be respectively used for different functionalities. For example, parts of the REs may be used respectively for transmitting a Cell specific Reference Signal (CRS), a user specific DeModulation Reference Signal (DMRS), a Channel State Information Reference Signal (CSI-RS), the PDCCH, the PDSCH, etc.
In the LTE system, for PDCCH transmission, a common search space and a UE specific search space are defined. The PDCCH is tracked through blind decoding of all sets of the common search space and the UE specific search space of a candidate PDCCH, and checking through Cyclic Redundancy Check (CRC) scrambled by a corresponding Radio Network Temporary Identifier (RNTI) (e.g., Cell-RNTI (C-RNTI)), to find the PDCCH scheduled by a base station. The number of candidate PDCCHs of the common search space and the UE specific search space is shown in Table 1. The common search space includes a search space with 4 Control Channel Elements (CCE) and a search space with 8 CCEs. The UE specific search space includes search spaces respectively with one CCE, 2 CCEs, 4 CCEs, and 8 CCEs. For each transmission mode, a UE needs to track two kinds of PDCCHs, which have different sizes of Downlink Control Information (DCI). Thus, for the common search space, the UE tracks 2*(4+2)=12 candidate PDCCHs. For the UE specific search space, the UE tracks 2*(6+2+2)=32 candidate PDCCHs.
TABLE 1Search Space Sk(L)The number ofSizecandidatekindAggregative level L[the number of CCEs]PDCCHs M(L)UE166specific21264828162Common41648162
CCEs occupied by a PDCCH search space Sk(L) areL·{(Yk+m)mod └NCCE,k/L┘}+i,i=0,1, . . . L−1
where L is an aggregation level, i.e., the number of the CCEs occupied by the search space;
k is an index of a subframe; and
NCCE, k is the total number of the CCEs in subframe k.
For the common space, Yk is equal to 0. Specifically, locations of the CCEs occupied by all UE common search spaces are same. For the UE specific search space, Yk is equal to (A·Yk-1)mod D, where Y−1=nRNTI≠0, A=39827, D=65537, nRNTI is a value of a user radio network temporary identity.
The UE specific search space is related to the value of the user RNTI and the index of the subframe. Specifically, after the value of the user RNTI is determined, a location of the CCEs occupied by the UE specific search space is determined.
In an LTE Advanced (LTE-A) system, overload of backward control signals and CRSs is reduced. At the same time, interference introduced by the backward control signals and the CRSs is decreased. Thus, UE frequency utilization is improved. Since the overload of the CRS is reduced, system power saving performance is further improved. PDCCH transmission and PDSCH transmission in the system are usually based on DMRS demodulation, which is referred to as New Carrier Type (NCT).
The PDCCH based on the DMRS demodulation is referred to as an Enhanced Physical Downlink Control CHannel (EPDCCH). A resource occupied by the EPDCCH is configured by a Radio Resource Control (RRC) signal. In an LTE release 11 system, the EPDCCH is only used for the UE specific search space, while the common search space uses the PDCCH. The EPDCCH is located in an EPDCCH set. The EPDCCH set is divided into a centralized EPDCCH set and a distributed EPDCCH set. The centralized EPDCCH set and the distributed EPDCCH set may occupy 2, 4 or 8 PRBs. Each UE configures 2 EPDCCH sets at most. Each EPDCCH is composed of 1, 2, 4, 8 or 16 Enhanced Control Channel Elements (ECCEs), while an ECCE is composed of 4 or 8 Enhanced Resource Element Groups (EREGs), and each PRB includes 16 EREGs.
The NCT may be divided into two kinds, based on whether it works as a stand-alone cell. For a non-stand-alone condition, the cell works only as a Secondary cell (Scell). For the secondary cell, the common search space is not needed to be configured based on current 3GPP specification. The common search space is configured in a Primary cell. The resource occupied by the UE specific search space composed of the EPDCCH may be configured by a high layer signal. Parameters required to be configured include whether the EPDCCH set used by the EPDCCH of the UE specific search space is centralized or distributed, the number of PRB pairs occupied by the EPDCCH set, and locations of the PRB pairs. Thus, the NCT may work normally as the Scell, where the NCT adopts the EPDCCH as the UE specific search space. For the stand-alone NCT, all LTE channels need to define an alternative technology in the NCT. Thus, an NCT cell may work normally.